Hi All,
really need some additional guidance/help...
Have been intermittently working on the rebuild of my old vespa...
This has involved...
Upgraded crank
New extended gear ratios
New DR 130cc kit
New electronic ignition system - BGM Pro 12v Touring
Sito+ exhaust
The carb is a Dellorto SHBC 19/19... with upgraded jets
I could not get the bike to run properly so took it to a guy i know...
He has sent me a message which i simply not sure how to proceed... See here below.
Any guidance much appreciated... weather starting to turn and gagging to get going!!!!...
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Hi Jeremy,
Apologies, just saw your email...
As it stands, I have absolute confidence in the motor, the compression is good, it revs out strongly and goes up and down through the gear box, the clutch works as it should .
The ignition system seems strong, the spark is good and it makes the motor run, I am a little concerned however that changing the ignition advance from fully retarded to fully advanced seems to have little to no effect – what should happen is with the timing fully retarded, the engine should be really easy to start but unwilling to rev out freely at the top end and lack power after the mid range, at the fully advance position the opposite is true, you end up with a slightly lump bottom end but free revving at the top.
You then adjust to suit to find the best possible middle ground – ie easy starting, good low and mid range and solid top end.
The issue I'm really struggling with is the fuelling.
The carb is now on the biggest pilot jet I can obtain for it, but the impact has been negligible.
The engine simply will not run with the airbox on AT ALL indicating it's far too restrictive for its new size and set up (which make sense in itself) and whilst I can get a pretty good mid range, the top end is lacking and the bottom end/idle is just not responding the way it should.
I have put her on our Dynapro dynamometer and the readings I'm getting however are totally sporadic, jumping from overly lean to far too rich – and adjustments made to the carb whilst on the fly are just not making the impact I would expect – even when I deliberately move from one end of the scale to the other.
An additional complication is that whilst the dyno can read 2 stroke motors perfectly well, I have to be VERY careful, as the sensor that's used in the exhaust is VERY sensitive and can become quickly damaged by 2 stroke oil – at a cost of £150 a go!
Normally, in a flat slide carb like yours, you would have a pilot jet which dictates the maximum amount of air that can get through the pilot (bottom end) circuit with an adjustable screw to taper that circuit as required, a main jet to dictate the maximum amount of fuel that can get though full stop, and a needle to moderate that – you don't want the maximum amount of fuel going in when only at say 50-75% throttle opening. What you do for the top end is get the main jet correct for wide open throttle, and use the needle to taper how far open the main jet is at anything below 100% throttle.
This carb, does not have a needle. Which is very odd and limits the available adjustment. This may also be why the readings jump so suddenly from one end of the scale to the other.
The other concern is that this carb as I understand it is a suitable replacement for the original, but of course this is not an original engine in original trim, all the parts fitted are with a view to greater volume in the motor, and greater acceleration and power.
I am of the opinion now that this carb simply isn't big enough nor adjustable enough to adequately and consistently fuel this motor. I have been looking into alternatives.
I hit road blocks at every turn with the manufacturers of the various parts, all of whom say different things and the opinions on what's best vary wildly.
It is quite the source of frustration as you can imagine...
I am currently hunting around for a compatible carb with a bigger bore and more conventional adjustment. There seem to be quite a number available but I do of course want to make sure what arrives is up to the task.
Interestingly, most of the carbs listed as suitable for bigger CC engines also come with a new intake tube, which makes sense as this would be a potential choking point were it not replaced at the same time, the other area of interest is what's referred to as "false air" – this is effectively an air leak that occurs between the carb and intake manifold which is likely to be exacerbated by the larger CC motor, and would in itself make any carb almost impossible to tune correctly as the volume of air should be tightly controlled by the carb itself, any air being drawn in other than via the carb causing serious running issues and inconsistency in the air/fuel ratio that reaches the motor.
I am of course happy to keep doing whatever it takes, can't say I've encountered one so challenging as this in many many years – but that's what happens when you work with old metal and use a Lot of interesting bits from various places, none of which were ever designed with one another in mind!
I would with your permission like to look at something like this:
https://www.sip-scootershop.com/en/product/carburettor-sip-dspc-19-round-slider_40025000?cr=3a6304bb-cc81-4ed5-9f63-1ff45fe1abfa&gad_source=1&gclid=CjwKCAjwkuqvBhAQEiwA65XxQLtKEePQnJAyEcWsfGq6mLpJm6RA-jkTt2PHHJK6Bnno-_AhqhbbqxoCJ_wQAvD_BwE
Hopefully that all makes sense, looking forward to hearing back from you soon – don't for one second think yours is ever over looked or ignored, it resides directly under my nose at all times!
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